1. Field of the Invention
This invention relates to optical beam processing through stimulated Brillouin scattering (SBS), and more particularly to beam combination and/or cleanup by directing one or more input beams along an extended length fiber.
2. Description of the Related Art
There is presently a need for continuous wave (CW) or long pulse lasers that are capable of producing a diffraction limited beam. A "diffraction limited" beam has minimum divergence, and is characterized by a planar wavefront and a Gaussian intensity profile. There are numerous industrial and medical applications for such beams. For example, welding lasers can benefit from a low divergence beam at the work place, since this allows for greater working distance and depth of field and thus allows for easier and less precise control of the welding equipment by the operator. Furthermore, the laser's output optics last longer when they are farther from the weld location.
At present only expensive and sensitive lasers are available to produce nearly diffraction limited beams. For example, there are presently no Nd:YAG lasers available that have diffraction limited beam divergence at high average powers. Even if a beam is nearly diffraction limited when originally generated, transmitting it through a multi-mode optical fiber will degrade its divergence and require a subsequent beam "cleanup" operation to restore its original low divergence. Furthermore, essentially all CW lasers are limited in the average power they can produce in a diffraction limited beam by practical or physical restraints. This includes helium-neon lasers, argon-krypton ion lasers, and diode lasers. It would be desirable to combine the outputs of many such lasers into a single powerful beam. While combining two beams has been successfully performed with the use of a polarizer, it is very difficult to combine larger numbers of beams.
SBS oscillators have been employed in the past to convert beams with poor divergence into more highly diffraction limited beams. When any material is penetrated by light with an intensity great enough to compete with the atomic forces that bind the material together, both the material and the light penetrating it are modified. This nonlinear interaction generates SBS time-reversed waves. With SBS the modified material generates sound waves that serve as reflective surfaces to produce the time-reversed waves. According to the Brillouin effect, a doublet is produced upon the scattering of monochromatic radiation, with the frequency of each of the two doublet lines differing from the frequency of the original input line by the same amount, with one line having a higher frequency and the other a lower frequency. SBS oscillators are a form of laser in which one beam is pumped by another to stimulate an oscillation between opposed mirrors. SBS oscillators are discussed, for example, in E. B. Aleksandrov, et al. "Stimulated Raman and Brillouin Scattering in Selective Resonators", Soviet Physics JETP 22, pages 986-992 (1966). Both SBS oscillator-amplifiers and stimulated Raman scattering (SRS, which has a much larger wavelength shift than SBS) have also been employed to improve beam divergence. Such devices are typically quite complex and employ SRS or SBS in bulk form.
In a related area, optical phase conjugation has been reported in short fiber waveguides, e.g., Serebryakov and Chertkov, "Wavefront Reversal of Microsecond Radiation in Fiber Waveguides", Soviet Journal of Quantum Electronics, vol. 17, 1987, pages 493-495. It has been considered important that the process be phase matched, or in other words that .DELTA.kL be less than .pi., where .DELTA.k is the difference in the propagation vectors of the relevant beams and L is the mutual interaction length of the input and return beams within the waveguide. This encourages the converted beam to be a phase conjugate of the pump beam if the device is an oscillator, or guarantees that the phase of the seed beam is imprinted upon the converted beam if the device is an amplifier. With the short fiber devices it is necessary to use higher peak power, pulsed lasers to operate sufficiently above the SBS threshold.
SBS has also been demonstrated in longer optical fibers, in which .DELTA.kL is substantially greater than .pi., since 1972; Ippen and Stolen, "Stimulated Brillouin Scattering in Optical Fibers", Applied Physics Letters, vol. 21, 1972, pages 539-541. Developments in this area through 1982 were reviewed by Cotter, "Stimulated Brillouin Scattering in Monomode Optical Fiber", Journal of Optical Communications, vol. 4, 1983, pages 10-19. Since then SBS has found use in optical amplification, although limited because of its restricted bandwidth; Tsubokawa and Sasaki, "Coherent FSK Transmission Experiment Using Brillouin Amplification in a Single-Mode Fiber", Journal of Optical Communications, vol. 10, 1989, pages 42-47. All of these investigations employed single-mode fibers, with a single-mode input beam propagated along the fiber and stimulating a single-mode return SBS beam. They have not been found to be useful in improving beam divergence.